Maybe he tried to show you that he appreciate your help by "wasting" his ammo/SL? ;D
And for real, there is always one who will try to break your day. But does it means we should not help each other?
I always try to help teammates, because even at the end of the day, if they turn out to be well, donkey, they are still part of the team (and can destroy enemy tank or lure him and give me opportunity to shot).
Of course it would be great to get random "thank you" via comm, but as Pacifica said - it's online game, expect no chivalry"